How do i replace oil pump on 2002 ranger 4 x 4 xtra cab 4-0 litre - driving at highway speed, lost oil pressure, stopped truck and restarted. Loud knocking but got off highway(about 1/4 mile). Got towed home. still starts but knocks loudly and oil pressure gauge jumps up and down.

Usually means the engine bearings are worn out, or the oil pump is weak.
As the oil gets hot, it gets thinner, which reduces pressure. When the engine is at idle speed the pump is not pumping as fast as it is at highway speed.

Factory gauges (especially Fords) are not known for being terribly accurate or reliable. As long as the gauge is pointed in the "normal" range, I wouldn't be too concerned.
If you want to pursue it, remove the oil pressure sending unit and perform an oil pressure test with a mechanical gauge. If the mechanical gauge fluctuates the same way, suspect a worn or failing oil pump. If the gauge is stable, I would suspect a faulty oil pressure sending unit.
If the truck has high miles, consider moving up to higher viscosity oil or adding an oil stabilizer. If could just be that the oil is breaking down when it's hot and the engine can hold consistent pressure with the thinning oil.

When your truck stops check for spark at the spark plugs, next check that you have fuel pressure at the shrader valve, If you have no spark check the ignition modular, if no fuel pressure test fuel pump PSI, if both are not present change the ignition modular in the distributor .

LOST OF OIL PRESSURE COULD BE WORN ENGINE BEARINGS OR FAULTY OIL PUMP.CHECK OIL LEVEL.IF ALL IS GOOD REPLACE OIL PUMP FIRST BEFORE DOING A ENGINE OVER HAUL.

sending unit may be bad. If engine is not knocking or tapping when pressure reads 0 likely that's the case.

Could be a dozen causes. Take it to a mechanic.

If you have checked the oil level and know it is full and not overfull, then you need to stop driving the truck until you solve this problem. It can be nothing more than a loose wire on the oil pressure sending unit. Or the sending unit can be bad. The gauge can also be bad. But is can also be a weak oil pump, or clogged oil pick-up screen. Considering it does this after it reaches normal operating temperature this is very suspicious and sounds like possibly engine wear. An oil pressure test and compression test will help diagnose the cause. You can just go ahead and check the wiring at the oil pressure sending unit and replace it if the wiring is good to eliminate that as a cause. If it is leaking it is for sure bad.

could be one of two things oil pump could be going bad or the main bearings are going bad no way of telling with out dropping pan and checking for lots of metal particles in the bottom of the pan if there is the main bearings are going bad.

If your oil pressure is running in the red this would indicate too much oil in the system. You should have your oil changed and refilled soon or you run the risk of burning out the oil pump. At idle your pressure should be just over a quarter of what your gauge has. At highway speed it should not exceed 3/4 of the gauge unless your going in excess of 100 MPH. Change the oil and filter and your vehicle should be fine.

Most likely your oil pressure relief valve located on in or nearby pump. Symptoms are oil pressure will drop when warmed up when idle lose all pressure eraticly giving the vehicle gas will bring back pressure and highway driving will bring pressure up until idle again.